1. Field of the Invention
This invention relates generally to digital control circuits which may be employed to control the speed and/or phase of rotation of a head or other element in a video tape recorder.
2. Description of the Prior Art
The prior art contains examples of analog servo control circuits for control of the speed and/or phase of a motor. Typically, the analog devices of the prior art trigger the generation of a ramp signal upon the occurrence of a predetermined event, such as the passage of a magnetic pole piece attached to the rotating element, whose speed and/or phase is to be controlled, past a stationary sensing or pickup coil to provide a sensed signal. A reference signal, which occurs after the sensed signal, is employed to sample and hold the value of the ramp signal existing at the occurrence of the reference signal. Since the ramp signal changes at a known rate, the amplitude of the ramp signal at any given time is proportional to the time after its initiation. The sampled and held analog value which is proportional to the time between the sensed signal and the reference signal is employed as a drive signal for a motor.
Such analog devices depend on the resistance-capacitance time constant of circuit components employed for generation of the ramp signal, and typically employ capacitance storage elements to perform the sample and hold function. Due to the resistance and capacitance tolerances of discrete resistors and capacitors, manual adjustment is required during the manufacture and maintenance of such devices, thus adding to manufacture and maintenance costs. In addition, the servo characteristics of analog circuits employing capacitance and resistance tend to vary due to the temperature coefficient and aging of such elements. Further, a system employing analog techniques cannot practically be made as an integrated circuit. Even when maximum integration is achieved, it is still necessary to externally connect discrete resistors and capacitors since capacitors in particular are not compatible with integrated circuit manufacture and the range of resistance values attainable is not acceptable. Consequently, manufacturing costs are increased and the temperature and aging variations noted in the preceding are not overcome. Furthermore, the need for assembly steps to install and adjust for such discrete externally connected parts makes integrated circuit design with such external components meaningless. Furthermore, if such an integrated circuit with discrete components were produced, the number of pins for connection to and from the integrated circuit chips is increased and this increase, combined with the size of the discrete components themselves, interferes with the desired high density circuit packaging.
Digital servo systems have been provided for controlling rotation of a rotary member, for example, by an electric motor. However, due to long-term changes in electric motors, electronic parts and the temperature characteristics thereof, a residual error may develop in a digital servo system, particularly in the phase control portion. Although theoretically this error could be minimized by making the servo loop gain infinite, such a solution is not practical.